The last fifty years a lot of effort has been made to provide a way to provide articles of e.g. steel with a surface layer of corrosion resistant material, such as tantalum. Tantalum possesses corrosion resistance even at high temperatures, so this material is advantageous in many fields where products are subject to corrosion. Further tantalum is a biocompatible and tissue-friendly material, so it is well suited for application in the medical field, such as for implants.
Known techniques for depositing refractory metals, such as tantalum, on an article, involve electroplating in an electrolyte comprising a mixture of fused salts, the mixture including a salt or salts of the refractory metal with which the article is to be plated. WO 98/46809 discloses a method for electroplating with a refractory metal. According to this method a molten electrolyte is used, said electrolyte consisting of refractory and alkali metal fluorides and a melt of sodium, potassium and caesium chlorides, and electrical current is passed through the electrolyte in alternating, repeating cycles. Methods for electroplating using mixed chloride-fluoride melts have, however, a tendency to result in porous coatings and coatings having a low adherence. Further caesium is very expensive, so therefore this method is not suited for industrial application.
EP 0 578 605 by the applicant in the present application discloses a fused-salt bath and a process for electrolytic surface coating. The bath comprises an alkali fluoride melt containing oxide ions and ions of the metal to be deposited.